@Dominik: Has anything changed regarding Active Sync implementation?
Are there now, 3 years later, plans to give the users a no-thrills-AS-account-setup on their mobile devices?

As long as CalDAV/CardDAV on iOS does not support push notifications ActiveSync is probably a nicer option to connect the client devices. Also you only have to set up one connection instead of two (compared to CalDAV/CardDAV).

This matters more than ever now that Google has started phasing out support for Exchange ActiveSync for their e-mail, contacts and calendar products.
http://www.wpcentral.com/google-drops-exchange-activesync-what-s-it-mean-windows-phone

None of the existing implementations could be integrated "as is" at our scale or really meets our quality standards.

If we're going to support ActiveSync, we have to build it ourselves.

But you're right, just split this ideas into two ideas (even they are related, because both would happen via ActiveSync). This idea tracks ActiveSync support for mobile devices.

Integration with other Exchange servers is now tracked in this idea:
http://fruux.uservoice.com/forums/153774-general/suggestions/2820044-sync-with-microsoft-exchange-server

well, sync with servers and sync with mobile devices are two different things - which one does matter more?
I think that since things like z-push and tine20 exist, it should be a matter of implementation rather than research to add support for syncing mobile devices via ActiveSync...